1. Field of the Invention
The present invention relates to a bi-directional overrunning clutched differential unit whose output shafts are freely, differentially rotatable from an input power unless the rotation speed of the output shafts becomes less than that of an input rotation as a result of the input power.
2. Related Art
As disclosed in US RE 38,012 E (hereinafter, “the '012 patent”) and US 2006/0254382 A1 (hereinafter, “the '382 Publication”), there are well-known conventional bi-directional overrunning clutched differential units. In each of these differential units, a differential housing supports a pair of coaxial output shafts (referred to as right and left output shafts), and the output shafts are freely, differentially rotatable from an input power unless the rotation speed of the output shafts becomes less than that of an input rotation as a result of the input power.
Each of the differential units includes a cylindrical cage disposed around the right and left output shafts. The cage holds right rollers aligned at regular intervals along the periphery thereof, and holds left rollers disposed leftward of the respective right rollers. In the differential unit of the '012 patent, a pair of right and left hubs are relatively unrotatably fitted on axially proximal end portions of the respective right and left output shafts. The axially proximal end portions of the right and left output shafts with the respective hubs thereon are relatively rotatably fitted into the cage, and in the cage, the axially proximal end portion of one output shaft is relatively rotatably fitted into the axially proximal end portion of the other output shaft. As a result, the right output shaft with the right hub thereon is relatively rotatably connected coaxially to the left output shaft with the left hub thereon. In the differential gear unit of the '382 Publication, the axially proximal end portions of the right and left output shafts with no hub thereon are fitted into the cage relatively rotatably to the cage and to each other.
In each of the differential units, a cylindrical clutch housing serving as an input rotary member is rotatably integrally provided thereon with a ring gear serving as an input gear, and is coaxially disposed around the cage. The clutch housing is formed on the inner peripheral surface thereof with cams corresponding to the respective rollers. Further, each differential unit includes a friction mechanism for applying a frictional rotation resistance onto the cage. As a result of the frictional rotation resistance, the rotation of the cage is delayed relative to the rotation of the clutch housing so that the rollers contact the respective cams. In this situation, when the rotation speed of the output shafts becomes less than the rotation speed of the clutch housing and ring gear, the rollers are wedged up between the respective cams and the hubs or output shafts, thereby engaging the overrunning clutch of the differential unit so as to transmit the input rotary force of the ring gear to the output shafts.
With regard to mechanisms for journaling the clutch housing, in the differential unit of the '012 patent, the clutch housing is journalled at only an axial intermediate outer peripheral portion thereof by the differential housing through a single bush. In the differential unit of the '382 Publication, the clutch housing (in this reference, “ring gear cage”) is journalled at one axial end thereof by the differential housing through a ball bearing, and at the other axial end thereof by the differential housing through a bush.
In the differential unit of the '012 patent, the single bush is axially wide, however, it is insufficient to prevent the rotating clutch housing from deviating. On the other hand, in the differential unit of the '382 Publication, the two bearings, i.e., the ball bearing and bush, journaling the axially opposite ends of the clutch housing are relatively advantageous in preventing deviation of the rotating clutch housing in comparison with the single bush journaling only the axially intermediate portion of the clutch housing as in the '012 patent. However, the differential unit of the '382 Publication is still insufficient in durability because it uses the bush.
With regard to mechanisms for journaling the output shafts, in the differential unit of the '382 Publication, the output shafts with no hub thereon project outward from the cage so as to be journalled by the differential housing through respective ball bearings. In the cage, the output shafts are independently separated from each other. Therefore, the decentering or torsion force of each of the output shafts that is insufficiently eliminated by each of the ball bearings is subjected to be transmitted to the rollers and the clutch housing. Consequently, the differential unit of the '382 Publication is insufficient in durability.
On the other hand, in the differential unit of the '012 patent, in the cage, the axially proximal ends of the output shafts are adjacent to each other, and a projection projecting from the axially proximal end of one output shaft is relatively rotatably fitted into the axially proximal end portion of the other output shaft so as to prevent the output shafts from decentering from each other during their rotation. On the outsides of the cage, the right and left hubs are journalled by the differential housing through respective ball bearings. However, when the axially proximal ends of the output shafts are adjacent to each other, lubricant oil filled in the differential housing is insufficiently supplied to splines which are formed on an peripheral surface of the axially proximal end portions of the respective output shafts, thereby causing the output shafts and hubs to be worn at their mutual meshing portions (splines) or to be rusted.